The invention relates generally to an apparatus for supporting foodstuffs in a microwave oven.
It is well known that electromagnetic energy can be utilized for heating foodstuffs or other lossy dielectric materials. The foodstuffs or other materials are placed in a cooking cavity of a microwave oven and are exposed to electromagnetic energy that is supplied by a suitable source, e.g., a magnetron. After a relatively short period of time during which the foodstuffs are subjected to electromagnetic energy, heat will be generated in the foodstuffs to accomplish the desired cooking of the foodstuff.
It is also well known that metal racks in a microwave oven may dissipate a substantial amount of the microwave energy supplied to the oven and also may distort the electric and magnetic field patterns in the oven, both effects thereby causing uneven cooking. In addition, arcing may occur between closely positioned elements of a metal rack in a microwave oven due to currents induced therein.
In U.S. Pat. No. 5,558,793 to McKee et al. (xe2x80x9cthe McKee ""793 Patentxe2x80x9d), assigned to the assignee of the present invention and incorporated by reference herein, a ceramic platter 64 (FIG. 3) is used to support foodstuff 12 in a hybrid oven 10 for cooking by hot air impingement and by microwave energy. Ceramic platter 64 includes a central aperture 66 which is used to channel the hot air around foodstuff 12, which, as described in further detail in the McKee ""793 Patent, ensures that both sides (i.e., top and bottom) of foodstuff 12 are cooked by the hot air flow. Ceramic platter 64 is substantially transparent to microwave energy. However, certain deficiencies exist in the use of ceramic platter 64. In particular, ceramic platter 64 is fragile and can suffer cracks or chips, especially when proper handling techniques are not followed. Such cracks or chips may allow moisture to be absorbed into ceramic platter 64 during washing. If ceramic platter 64 is used before the absorbed moisture evaporates, it can fracture. Also, since water absorbs microwave energy, the water absorbed into the cracked or chipped ceramic platter 64 affects the microwave energy pattern in the oven, resulting in uneven energy patterns that detrimentally affect the cooking of foodstuff 12 until the moisture evaporates. In addition, although ceramic platter 64 is only moderately expensive, frequent replacement thereof can cause the accumulated cost to rise to an unacceptable level. Finally, over time ceramic platter 64 may absorb grease and accumulates a carbon buildup on its surface, both of which reduce the usable life span of ceramic platter 64.
It is well known that pressed paperboard trays and other types of non-metallic food carriers may be used to support foodstuffs in microwave ovens. Pressed paperboard trays, e.g., Pressware(copyright) Classic Stoneware Trays from Pressware International, Inc., are much less expensive than the ceramic platter disclosed in the McKee ""793 Patent. However, such pressed paperboard trays present certain deficiencies when used in the hybrid oven of the McKee ""793 Patent. In particular, such platters may deform due to lack of structural support when subjected to the impingement hot air flow in such an oven. Other types of food carriers which are microwave transparent such as baking paper or teflon-coated fiberglass mesh suffer similar deficiencies.
Accordingly, an object of the present invention is to provide such apparatus for supporting foodstuffs, either directly or on a pressed paperboard tray, in a hybrid oven.
An additional object is to provide such apparatus which in an embodiment supports foodstuffs, either directly or on a pressed paperboard tray, in a hybrid oven and which is relatively transparent to microwave energy so that it does not significantly affect the pattern of microwave energy supplied to such foodstuffs.
Yet another object is to provide such apparatus which in an embodiment supports foodstuffs, either directly or on a pressed paperboard tray, in a hybrid oven and which, in a preferred embodiment, ensures that the air flow is directed along both sides of such foodstuffs.
It is yet a further object to provide such apparatus which in an embodiment supports foodstuffs, either directly or on a pressed paperboard tray, in a hybrid oven which apparatus is more cost-effective than conventional supports.
It has now been found that the above and related objects of the present invention are obtained in an apparatus for supporting foodstuffs in a microwave oven for cooking by microwave energy having a predetermined free-space wavelength. The apparatus of the present invention may also be used in a hybrid oven which combines cooking by hot air impingement with cooking using microwave energy. In a first, preferred, embodiment, the apparatus comprises a continuous outer peripheral member formed of metal, and a plurality of primary spoke members formed of metal, rigidly secured to the continuous outer peripheral member in substantially equally spaced circumferential intervals, and projecting centrally inwardly on a substantially common plane with or above the continuous outer peripheral member. The primary spoke members have a length which is an integral multiple of one-half of the free space wavelength of the microwave energy of the hybrid oven and which is less than one-half of the shortest distance between two diametrically opposite points on said substantially continuous outer peripheral member. By setting the length of the primary spoke members to an integral multiple of one-half of the free space wavelength of the microwave energy, the apparatus minimizes the disruption of the microwave energy pattern in the hybrid oven.
Preferably, the apparatus also includes a metal plate mounted to the primary spoke members. The metal plate preferably has a disk configuration with an outer diameter substantially equal to that of the continuous outer peripheral member. The metal plate also has a central aperture, preferably circular and having an inner diameter that is slightly larger than the diameter of the microwave launcher opening within the microwave oven. An opening is formed at the center of the apparatus by the central aperture of the metal plate.
In addition, the apparatus preferably also includes a plurality of secondary spoke members, equal in number to the plurality of primary spoke members, formed of a metal and rigidly secured to the continuous outer peripheral member in substantially equally spaced circumferential intervals to cooperate with the primary spoke members to support either a microwave transparent tray holding the foodstuffs placed thereon or the foodstuffs themselves. Each of the secondary spoke members is rigidly secured to the continuous outer peripheral member at a point which substantially bisects the angle formed on the continuous outer peripheral member by adjacent pairs of the primary spoke members. The plurality of secondary spoke members project centrally inwardly on a common plane with the continuous outer peripheral member. Each of the secondary spoke members has a length which is an integral multiple of one-half of the free space wavelength of the microwave energy of the hybrid oven and which is less than one-half of the shortest distance between two diametrically opposite points on said substantially continuous outer peripheral member. Setting the length of the secondary spoke members to an integral multiple of one-half of the free space wavelength of the microwave energy also minimizes the disruption of the microwave energy pattern in the hybrid oven by the apparatus of the present invention.
In a preferred embodiment, the continuous outer peripheral member forms a ring and has a predetermined diameter. The length of the primary spoke member is 1.5 times the free space wavelength, and the length of the secondary spoke member is 1.0 times the free space wavelength. In addition, the primary spoke members have an inverted xe2x80x9cTxe2x80x9d cross-section, thereby forming a support for either a microwave transparent tray holding the foodstuffs placed thereon or the foodstuffs themselves.
In a preferred embodiment for use with microwave energy of 2450 MHZ, the diameter of the continous outer peripheral member is 44.2 cm (17.4 inches), the length of the primary spoke members is 18.3 cm (7.2 inches), the length of the secondary spoke members is 12.2 cm (4.8 inches) and the diameter of the center aperture formed by the inner diameter of the metal plate ranges between 24.13 cm (9.50 inches) and 25.40 cm (10.00 inches).
In a second embodiment, the apparatus of the first embodiment is mounted in an aperture in flat tray formed from a microwave transparent material, such as a ceramic glass. The apparatus of the second embodiment allows the present invention to be used in applications which only accept rectangular racks.